We propose to analyze the cytological and molecular mechanisms of the eukaryotic cell cycle by utilizing various mutant strains and developmental states of Saccharomyces cerevisiae. In this species the spindle plaques play a central role in controlling cell division and coordinating events of the cell cycle. Structural modification of these spindle plaques and their attached microtubles during mitosis, meiosis, and karyokinesis will be determined by kinetic studies and examination of cell division cycle mutants. A system for the initiation of polymerization of exogenous tubulin in vitro will be used both for analysis of the mechanisms controlling microtubule distribution and for purification and molecular characterization of the spindle plaques.